The unopened eyes of some newborn mammals might be more sensitive to light than was previously thought, according to a report by Akerman et al. in Neuron.

It was already known that very bright or high-contrast stimuli can elicit a response in the visual system before eye opening — for example, the eyes of a ferret can respond to light as early as postnatal day (P) 19, even though they do not open until around P32. However, the effects of natural levels of light on the unopened eye have not been explored until now.

Akerman et al. presented young ferrets (P20–P26) with movies that simulated the types of visual stimuli that they would be exposed to in their normal rearing environment. They measured the activity of neurons in the lateral geniculate nucleus (LGN), which relays visual information from the retina to the visual cortex. Surprisingly, instead of seeing random spontaneous activity, the authors found that the LGN neurons showed patterns of activity that were consistent with a specific response to the light stimulus. This indicated that the retinal ganglion cells (RGCs) that project to this nucleus were transmitting meaningful information.

This study also shed new light on activity-dependent processes during visual system development. The authors examined the development of projections to the LGN from two types of RGC — 'On' cells, which respond to increases in light intensity, and 'Off' cells, which respond to decreases in light intensity. Usually, during early postnatal development, the On and Off LGN afferents segregate, so that individual LGN neurons come to receive inputs from only one cell type. However, in ferrets that were reared in the dark from P16, some LGN neurons were found to respond to both increases and decreases in luminance. This implies that light is required for the normal segregation of the On and Off afferents.

The role of visual experience in the early sculpting of the visual system is still controversial, and this report is likely to add further fuel to the debate. There is little question that neuronal activity has a role, but the events that occur before eye opening were assumed to be driven by spontaneous activity, rather than stimulation of the nervous system by light. These new observations raise the possibility that some of the effects that were formerly attributed to spontaneous activity might actually result from visual experience.